This invention relates to the packaging of semiconductor devices, and particularly to packaging micromachined accelerometers.
Glass sealed ceramic packages have been used in the IC industry for a number of years and in a number of applications, including for packaging micromachined accelerometers. In these packages, a die is mounted in a container, e.g., with a silver-glass paste. The container has an open top and a lid, each of which has a mating glass layer. Leads are placed across the glass layer of the container. The container with the device is heated in air to 400-450xc2x0 C. to sinter the inorganics, volatilize the organics in the paste, and embed the leads in the glass layer on the container. The leads are wire-bonded to bond pads on the device. The lid is placed over the container, and the assembly is again heated to 400-450xc2x0 C. in air to seal the packages. In this latter heating process, the seal glass layers soften and fuse to form a hermetic seal. A process of this general type (but with some differences) is described in U.S. Pat. No. 5,736,607, which is expressly incorporated herein by reference for all purposes.
A surface micromachined accelerometer has closely spaced mechanical structures. These structures have high energy surfaces that will tend to stick together if they come in contact, in which case the device may be rendered nonfunctional. One way to reduce surface energy and to passivate the surface to prevent such stiction is to deposit a thin organic coating, as described in U.S. Pat. No. 5,694,740, which is expressly incorporated herein by reference for all purposes. In one embodiment, for example, a small amount of an organic material is provided in a container. The material vaporizes and is thus deposited on the micromachined structure during the heating/sealing processes. Few organic materials suitable as coatings can survive heating processes performed in air for hermetically sealing containers. Such temperatures may currently be as low as about 300xc2x0 C., and often above 400xc2x0 C.
The present invention includes a process for hermetically sealing a semiconductor device, either at the wafer level with a cap over the device or a portion of the device, or in sealing a device in a container, such as a cerdip or cerpac. According to the present invention, the device may be sealed at the wafer level with a cap or in a container in a reduced oxygen environment that has a nonzero amount of oxygen that is less than the 21% oxygen found in ambient air (referred to as a xe2x80x9creduced oxygen environmentxe2x80x9d). The amount of oxygen is preferably 0.1%-15%, more preferably 1%-10%, still more preferably 2%-4%, and most preferably about 3% oxygen.
The device is preferably a semiconductor device that has a movable component that is used as a sensor, more preferably a micromachined accelerometer, and still more preferably a surface micromachined accelerometer. The accelerometer preferably has a mass suspended over and parallel to a substrate, tethered to be movable along a sensitive axis, and having fingers extending away and between fixed fingers to form differential capacitive cells.
The method of the present invention is further preferably used in conjunction with a process for sealing a semiconductor device with a movable (relative to a substrate) component in which an organic material, in solid, liquid, or vapor form is introduced into a package and heated to coat the movable component to prevent or reduce the risk of stiction in the device or to control electrical characteristics. Alternatively, the device may be coated with the organic material before the heating and sealing.
Another method of the present invention is the use of a reduced oxygen environment when hermetically sealing an organic component, particularly when the seal is formed from glass, in order to reduce the oxidation of an organic component, while providing a non-zero quantity of oxygen to ensure that the seal is formed properly (in the case of glass, so that the glass wets to form a strong and reliable hermetic seal). The amount of oxygen is preferably 0.1%-15%, more preferably 1%-10%, still more preferably 2%-4%, and most preferably about 3% oxygen.
In the case of an organic coating applied directly on a device in small quantities, e.g., by evaporating or through other semiconductor processes, rather than introducing drops of liquid and vaporizing, the package may be sealed with a small amount of oxygen, e.g., less than 4%, down to an oxygen-free environment because there is less organic material that can affect the quality of a hermetic seal.
Other features and advantages will become apparent from the following detailed description, drawings, and claims.